The present invention relates to a semiconductor pressure sensor and more particularly to a semiconductor pressure sensor having a container which forms a reference pressure chamber, a semiconductor pressure sensitive diaphragm to which an external pressure is applied on the side opposite to the space inside the container, and a circuit substrate on which a circuit connected to the strain gauge of the pressure sensitive diaphragm is mounted.
One type of pressure sensor, proposed by the present applicant et al. in Japanese Patent Application No. 71021/1980, attempts to improve the mechanical and thermal stabilities of a conventional pressure sensor, and consists of a resin mold, a container which house only a pressure sensitive diaphragm, and wherein an amplifier circuit and other components are installed on a circuit substrate provided outside the container. An example of this is shown in FIG. 1, in which a metal package, for example MD18 or equivalent, consisting of a metal cap 1 and a metal substrate or stem 2 which may also be used in a semiconductor device, is used as a container of the sensor. The stem 2 is provided with a central opening 3, in which a cylindrical pedestal 4 of silicon is disposed. The pedestal 4 is centrally provided with a through hole 5, over which a silicon pressure sensitive diaphragm 6 is fixed such that it plugs up the hole. The pedestal is joined to a pressure guiding pipe 7 so that the through hole 5 is in communication with the inside of the pipe 7, which is supported by the stem 2 via a metal cap 8. A ceramic substrate 9 is fixed on the upper surface of the stem 2 so that it surrounds the pressure sensitive diaphragm 6. On the ceramic substrate 9 a thick film circuit (not shown) is formed, and operational amplifier chips, diode chips, and other components are installed through the thick film circuit to form amplifier circuits and adjustment circuits, which are connected to the bridge of a strain gauge formed on the pressure sensitive diaphragm 6 through thin bonded lead wires 10.
The outputs of the adjustment and amplifier circuits are connected to a post 11 via a thin bonded lead wire 12. This post 11 is insulated from the stem 2 by an hermetic seal. As such, the output of the sensor is available outside the container through an output lead line 13 connected to the post 11. The output depends on the difference between a pressure undergoing measurement, that is guided to one side of the pressure sensitive diaphragm 6 from the pressure guiding pipe 7 through the through hole 5 in the pedestal 4, and the pressure in the reference pressure chamber 14 within the cap 1. Therefore, by setting the pressure in the reference pressure chamber 14, one can set the sensor output level. For protecting the pressure sensitive diaphragm, the thick film circuit on the ceramic substrate or the semiconductive chip within the container, helium or inert gas containing helium is admitted through a sealing pipe 15 until the level of the sensor output reaches a predetermined value, whereupon the admittance of the gas is stopped and the pipe 15 is sealed off, thus effecting the aforementioned setting.
In the sensor, if iron having a thermal expansion factor of 11.7.times.10.sup.-6 is used as the material of the stem 2, for example, then the difference between the expansion factor of iron and that of the ceramic substrate 9 (7.times.10.sup.-6) will cause a stress to be produced when the substrate 9 is brazed to the stem 2 with soft solder. This stres will cause the resistances of the resistors printed on the substrate 9 to be affected, and a resistance variation on the order of 1% may sometime occur. In order to prevent generation of such a stress, it has been suggested that the substrate 9 be partially brazed to the stem 2 with soft solder. However, since the adhesive strength between a metal and a ceramic is generally weak when soft solder is used, a partial joining in only a relatively small area is insufficient to create adequate adhesion, and a relatively large area of joining is therefore required. It has also been suggested to employ an adhesive which does not need heating much different from soft solder, but such an adhesive produces some problems including insufficiency of the adhesion and generation of a stress in the substrate 9 caused by temperature change during use.